The present invention generally relates to rechargeable cells that demonstrate, among other things, good cycle-life, low after-cycle gassing and improved cumulative discharge capacity and, more particularly, relates to rechargeable cells that employ a mercury- and lead-free zinc-bismuth alloy as the negative active material.
Batteries are used to power a wide variety of products and basically constitute electrochemical systems or devices which are capable of converting electrochemical energy into direct current. Examples of such electrochemical systems include AgO/Zn, Ag2O/Zn, HgO/Zn, HgO/Cd, Mn2O/Zn, Ni/Zn, Ni/Cd, Ni/MH and Zn/air.
Batteries are made up of one or more battery cells. Each battery cell basically comprises a cathode electrode, an anode electrode, a battery separator and a quantity of an electrolytic solution. Upon application of an electrical potential or load across the cell, electrons are generated through oxidation at the anode. The generated electrons pass through the load and then return to the cell at the cathode, where the cathode is reduced.
Batteries are either primary or secondary. Primary batteries produce energy by consuming one of the chemicals employed in the one or more cells that make up the primary battery. When the chemical is gone, the primary battery no longer produces energy and must be replaced. Secondary or rechargeable batteries obtain energy by transforming certain kinds of chemicals. When the transformation or change is complete, the battery no longer produces energy. It can be renewed or recharged, however, by sending current from another source through the battery to restore the chemicals to their original state.
The performance requirements or expectations of primary and secondary batteries are fundamentally distinct. Primary batteries are expected to exhibit low self discharge rates and to satisfy demanding performance requirements. Secondary batteries, on the other hand, are expected to demonstrate good cycle life and cumulative performance. As will be readily appreciated by those skilled in the art, rules and parameters that govern self-discharge rates and performance of primary batteries differ from those that govern cycle life and cumulative performance of secondary batteries. It would, therefore, not be feasible to attempt to predict the effect of a change in, for example, the electrode make-up, on the performance of a secondary battery from the effect of such a change on the performance of a primary battery.
Primary and rechargeable manganese dioxide-based alkaline cells are well known and include a cathode or positive electrode having manganese dioxide (MnO2) as an active material, an anode or negative electrode utilizing zinc as an active material, an aqueous solution of potassium hydroxide as electrolyte, and a separator between the positive and the negative electrodes.
The zinc electrodes that are employed in rechargeable or secondary alkaline zinc batteries and cells, such as rechargeable alkaline manganese (RAM) cells, are made from metallic zinc (Zn) or zinc oxide (ZnO). Metallic zinc is typically used when charged positive electrodes are readily available, such as MnO2 and air electrodes. Such electrodes can be charged electrically or refueled (e.g., pumped out, electrically recharged and pumped in). Otherwise, the negative electrode in rechargeable zinc cells is typically a porous, polymer-bonded Znxe2x80x94ZnO sheet, that has to be electroformed. In the latter systems, additives (e.g., mercury, cadmium, lead, indium, bismuth and the like) can be employed in the form of oxide or hydroxide compounds in amounts ranging from 1 to 5 wt. % to control hydrogen gassing, shape change and dendrite growth of the secondary zinc electrode. For environmental reasons, however, the use of mercury, cadmium and lead is minimized or avoided.
Secondary alkaline zinc batteries and cells including RAM cells consistently demonstrate poor cycle-life reportedly due to zinc redistribution and the high solubility of the zinc electrode discharge product in strong alkaline electrolytes. These factors reportedly contribute to or cause observed shape changes, gassing and formation of dendrites. As a result, it has been very difficult to produce sealed rechargeable cells with zinc electrodes without providing a resealable venting mechanism that would release excessive gassing developed during cycling and storage.
Effects of alternative alloying elements on RAM performance were discussed in Symposium on Rechargeable Zinc Batteries, Electrochemical Society 1996, PV95-14, p120. This article discloses that lead-free indium-zinc (Inxe2x80x94Zn) alloy powder could be used in mercury-free RAM cells. This article does not, however, disclose or discuss material utilization and/or cumulative capacity of RAM cells utilizing bismuth-zinc powders.
Several patents teach the use of zinc alloy powders as the negative active material in primary alkaline manganese (PAM) cells. As alluded to above, however, primary and secondary batteries and cells represent separate and distinct arts due to the fundamentally distinct performance requirements assigned to these devices.
For example, U.S. Pat. No. 5,721,068 describes a cell that comprises zinc or a zinc alloy containing zinc and bismuth in the anode for primary batteries. Although reference to rechargeable zinc batteries is made in this reference, no information on cycle-life is provided.
In view of the above, it is a general object of the present invention to provide a rechargeable cell or battery that serves to address the above-mentioned problems or deficiencies attributed to prior art rechargeable cells or batteries.
It is a more specific object to provide a sealed, rechargeable cell or battery that can operate without a resealable venting mechanism.
It is an even more specific object to provide a RAM cell that demonstrates good cycle life, low after-cycle gassing and improved cumulative discharge capacity.
It is an even more specific object to provide a RAM cell that demonstrates improved high drain performance.
It is another object to provide an anode for use in rechargeable cells that employs a zinc alloy as the negative active material.
It is yet a further object to provide a zinc alloy material for use as a negative active material in rechargeable cells.
The present invention therefore principally relates to a zinc alloy powder for use in rechargeable cells, which comprises from about 50 to about 170 ppm bismuth and, optionally, from about 0.003 to about 0.2% by weight (wt.) indium.
The present invention also relates to a zinc-bismuth alloy powder as defined hereinabove that contains 0.1% by wt. or more of particles having an average particle size ranging from about 20 to about 45 microns (xcexc)
The present invention further relates to an anode that employs the above-referenced zinc alloy powder as the negative active material.
The present invention also relates to a rechargeable cell that basically comprises a positive electrode, an anode utilizing the zinc alloy material as defined hereinabove, a separator and an alkaline electrolyte, all in a suitable container.
The foregoing and other features and advantages of the present invention will become more apparent from the following description.